BSSAP

SS7 protocol suite

SS7 protocols by OSI layer
Application	INAP, MAP, IS-41... TCAP, CAP, ISUP, ...
Network	MTP Level 3 + SCCP
Data link	MTP Level 2
Physical	MTP Level 1

BSS Application Part (BSSAP) is a protocol in Signaling System 7 used by the Mobile Switching Center (MSC) and the Base station subsystem (BSS) to communicate with each other using signalling messages supported by the MTP and connection-oriented services of the SCCP. For each active mobile equipment one signalling connection is used by BSSAP having at least one active transactions for the transfer of messages^[1]

BSSAP provides two kinds of functions:

• The BSS Mobile Application Part (BSSMAP) supports procedures to facilitate communication between the MSC and the BSS pertaining to resource management and handover control.
• The Direct Transfer Application Part (DTAP) is used for transfer of those messages which need to travel directly to a Mobile equipment from MSC bypassing any interpretation by BSS. These messages are generally pertaining to Mobility management (MM) or Call Management (CM).

References

1. 3GPP TS 48.008 Mobile Switching Centre - Base Station System (MSC-BSS) interface; Layer 3 specification